At the end of a day when political news was dominated by a former London mayor further wrecking his own and his party’s reputation, the outline of the good a Labour successor could do took shape in the Copper Box arena on the Queen Elizabeth Olympic Park.

Sadiq Khan had begun the day as one of the first to denounce Ken Livingstone for his “appalling and inexcusable” remarks about Hitler and antisemitism and ended it mobbed by admirers drawn from just about every ethnic and religious group in the city after addressing a crowd of 6,000 London Citizens campaigners.

The Citizens’ Mayoral Accountability Assembly is a regular highlight of London mayor campaigns. A mass gathering of primarily faith group activists, Christian, Jewish, Muslim and more, its fervour and high emotion can both exhilarate and intimidate even seasoned politicians as formidable clerics and youthful idealists politely but sternly invite them to sign up to their goals.

It was, in fairness to his laid back Conservative rival Zac Goldsmith, an occasion made for the combative, energetic Khan. A religious, socially liberal man able to greet his audience in both ecumenical and Muslim styles, his policies on housing, stridency on low pay and stance on citizenship fitted the Citizens’ requirements more precisely than those of Goldsmith, who had little choice but to finesse his less compliant offers as best he could.

After young refugees had spoken of their yearning to remain in the city where they had grown up, Khan pointed out, to loud cheers, that in his role as Tooting MP he had helped try to amend the government’s immigration bill to force it to help more unaccompanied children stranded in mainland Europe. That is why he missed the National Housing Federation hustings on Monday. Goldsmith’s excuse, according to his stand-in that evening, was that he had to meet someone in Hillingdon.

At the end of the Assembly, Goldsmith left quietly in the company of his campaign chair, the former Enfield North MP Nick de Bois, and almost no one else. Not for the first time, he gave the impression he’d sooner be in Devon sympathising with independent cheesemakers than revelling in the capital’s polyglot. Meanwhile, on the other side of the venue, the diminutive Khan disappeared into a crush of well-wishers, camera crews and selfie-seekers. He’d empathised. He’d rocked the house.

No event on the long and increasingly poisonous campaign trail had captured with the same emotional clout the overwhelming case for Khan rather than Goldsmith to become London’s next mayor on 5 May. Naturally, the Tories, led by the increasingly risible Boris Johnson, are straining to stick some of the muck stirred by Livingstone to Khan. They can point out that Khan was pleased to receive the backing of Livingstone when seeking to become the Labour candidate. They are not, of course, mentioning that his endorsement received equal billing with that of Oona King, the former Bethnal Green MP who is part Jewish or that he was also supported by Margaret Hodge, the daughter of Jewish refugees.

A new Survation poll, conducted earlier this week, has given Khan a 20 point lead over Goldsmith - exactly the same massive margin found by YouGov last week. If both those polls - and every other poll conducted this year - turn out to have been wildly wrong, it will be sensational. Yet no one watching this mayoral race closely expects Khan to win by a huge distance. And the Livingstone mess, which took place after Survation had completed its survey, cannot have helped him.

What, if any, impact might it have? Robert Shrimsley begins his column in the Financial Times (paywall) by describing it as an apology to Khan, for whom he was intending to vote. He explained that despite his dislike for Goldsmith’s divisive tactics and his preference for Khan - who he said could be “a key figure in efforts to promote greater harmony between the Muslim and Jewish communities” - he could no longer support him because “a vote for Khan is a vote for Labour,” at a time when Labour under its leader Jeremy Corbyn, criticised for his handling of the antisemitism row, “has shown itself to be a party that is at best indifferent to antisemitism and at worst hostile to the Jewish community.”

Shrimsley is concerned that a Khan win would help preserve Corbyn’s leadership. I respect his feelings and those of others who share them. Even so, I urge them to reconsider what a Khan victory would mean. I wrote a while ago that even though a Khan win in London would give comfort to Corbyn on a night when Labour is expected to suffer in other elections across the UK it would also expose Corbyn’s failings, not least as a builder of bridges to reach people who might otherwise reject him.

I included in these Jewish Londoners, whom Khan has eagerly courted ever since becoming Labour candidate. His efforts have won him deserved plaudits. Khan would not be “Corbyn’s man in London,” as Goldsmith’s devious campaign falsely claims: that isn’t how the mayoralty works. Rather, he would be a beacon of autonomy as the most powerful Labour politician in the land. Khan is not part of Labour’s antisemitism problem. He is already part of the antidote to it. As London mayor, he would become a hugely visible and influential one - and a national rallying point for how Labour ought to be.